Even though looking up an element of a hash tends to be an O(1) operation, its algorithmic complexity only describes the scaling factor of the algorithm as n increases. If you rearranged the if/else branches in decreasing order of probability according to your expected corpus, it would be faster than the hash lookup until you reached (wild guess) a dozen or so entries in the hash, or if your working corpus deviated significantly from your expected corpus.

A good benchmark with working example data would reveal more, but remember that:

This depends on your version of Perl, your workload, and your machine.

Getting the best performance possible is a matter of tuning.

Perl is sensitive to fluctuations between different optrees and working sets.

The slow bit is the function call, not the hash lookup which you'd also have to do with the symtab anyway. With a hash, you have better control over what inputs are allowed and how they are dispatched.